Radio systems used for communications include a transmitter and a receiver. The transmitter sends an electrical signal to a radiating element (antenna), which facilitates the propagation of the signal as electromagnetic waves that can travel through free space. The electromagnetic wave can be intercepted by a receiving antenna (receiver), which captures some of the energy of the wave. The receiver converts the captured energy into a received signal that can be interpreted to recover the information conveyed from the transmitter.
This disclosure relates to “near-field” communication systems, as opposed to “far-field” radio systems. The distinctions between near-field and far-field systems relate to the distance separating the transmit and receive antennas. Assuming a transmitting antenna with a largest dimension D transmitting a signal of wavelength λ, for example, the receive antenna is in the near field if the distance separating the transmitting and receiving antennas is less than 2D2/λ. Larger separations place the receive antenna in the far field. The distinction between the near and far fields is important because some components of a transmitted signal fall-off rapidly with distance. The properties of transmitted electromagnetic waves therefore change dramatically between the near and far fields.
Near Field Communication (NFC) is a wireless technology that supports communication within the near field region. Typical uses include RFID, contactless payment methods, identity documents, and electronic keys, all of which are low data-rate applications. More recently, NFC is a candidate for mobile-to-mobile communication, and for communication within electronic products. Such wireless interconnection technologies often require higher data rates than traditional NFC systems.